1. Field of Invention
Various systems such as fuel cells and air separation plants to generate oxygen or nitrogen require highly efficient, compact and oil free motor driven compressors to compress working gas or air to a higher pressure level. The field of invention pertains to conception, design and manufacturing of small motor driven compressors, usually less than 10 kW, and associated technologies including their integration required for fuel cell systems for transportation, small power plants, and small air separation plants including one installed on an aircraft.
Historically, compressed gas/air has been generated by various types of motor driven machines, e.g. piston, screw, vane, and centrifugal etc. To achieve high efficiency the motor must drive centrifugal compressor/rotor at high rotative speeds. As rotative speeds become greater the overall machine size can be made smaller, while maintaining the same compressed gas/air flows and pressures. Requirements for running at high speeds include properly designed rotating and non-rotating assemblies and bearings to support the high speed rotating shaft, typically 30,000 rpm to 200,000 rpm.
2. Related Art
High speed turbine driven rotating machines supported on foil air bearings have made significant progress during the last 30 years. Reliability of many high speed rotating machines with foil bearings has shown a tenfold increase compared to those with rolling element bearings. Most high speed rotating machines are Air Cycle Machines (ACM) used in Environmental Control Systems (ECS) of aircraft that manage cooling, heating and pressurization of the aircraft. Today, ACM for almost every new ECS system on military and civil aircraft and on many ground vehicles use foil air bearings. Old ECS systems with rolling element bearings are being converted to foil air bearings. The F-16 aircraft ACM used rolling element bearings from 1974 to 1982, but all aircraft built since 1982 use foil air bearings. The 747 aircraft ACM used rolling element bearings from 1970 to 1989. All aircraft built since 1989 have foil air bearings. ECS on the older model 737 aircraft have rolling element bearings, whereas ECS on new 737 use foil air bearings. An overview of foil bearing technology is provided in an ASME paper (97-GT-347) by Giri L. Agrawal.
The use of foil bearings in turbomachinery has several advantages:
Oil Free Operation—There is no contamination with oil. The working fluid in the bearing is the system process gas which could be air or any other gas. For many systems such as fuel cells oil free operation is a necessity.
Higher Reliability—Foil bearing machines are more reliable because there are fewer parts necessary to support the rotative assembly and there is no lubrication needed to feed the system. When the machine is in operation, the air/gas film between the bearing and the shaft protects the bearing foils from wear. The bearing surface is in contact With the shaft only when the machine starts and stops. During this time, a coating on the foils limits the wear.
No Scheduled Maintenance—Since there is no oil lubrication system in machines that use foil bearings, there is never a need to check and replace the lubricant. This results in lower operating costs.
Environmental and System Durability—Foil bearings can handle severe environmental conditions such as shock and vibration loading. Any liquid from the system can easily be handled.
High Speed Operation—Compressor and turbine rotors have better aerodynamic efficiency at higher speeds. Foil bearings allow these machines to operate at the higher speeds without any limitation as with ball bearings. In fact, due to the hydrodynamic action, they have a higher load capacity as the speed increases.
Low and High Temperature Capabilities—Many oil lubricants cannot operate at very high temperatures without breaking down. At low temperature, oil lubricants can become too viscous to operate effectively. Foil bearings, however, operate efficiently at severely high temperatures, as well as at cryogenic temperatures.
The air cycle machines described above are turbine driven. The motor driven rotating machines require various additional technologies for operation. They are:
The foil bearings must have higher spring rate to compensate for negative spring rate for the motor rotor.
More cooling flow is required between rotor shaft and the stator to cool the additional heat generated by motor.
An effective cooling scheme is required for the high speed motor stator.
Motor material cannot handle tensile stress generated by bending.
The centrifugal compressor should not surge under normal low flow condition which could be approximately 10% of the design flow resulting in 20% of the design speed.
Motor driven machines will be longer than turbine driven machines. Hence bending critical speed should not create problem.
Controller should provide high frequency required for the high operating speed.